Screening for explosives at checkpoints is a high priority for a number of Government agencies. Depending on the checkpoint, this screening can involve the search of people, hand-carried items, shipped or mailed items, and vehicles. Of these, the screening of vehicles for explosives is the most difficult due to the number of places that explosives may be hidden in a vehicle, and very important due to the large amount of explosives that may be carried in a vehicle.
Current methods of screening vehicles for explosives includes physical search by people, the use of trained animals (such as dogs), x-ray portals, portals based on nuclear technologies, and surface swipes. Each of these methods has significant limitations.
Physical search by trained people requires little equipment, but has high labor costs. It is also quite slow and likely to miss explosives hidden within body panels or buried in large loads. The physical search is most useful as a follow-up to verify or disprove an indication of explosives by one of the other methods.
The use of trained dogs is effective and relatively quick, but dogs have a durability problem in that they require breaks every couple of hours, and they require a handler, which is a high labor cost. They would not be acceptable at a high-traffic checkpoint, although they are very useful as a backup to an automated inspection system.
An x-ray portal can see behind panels and into tires; a great advantage in detection. However, these portals are very expensive and require the occupants to leave the vehicle while it is under test. A typical inspection would also be too slow for a high traffic checkpoint. Furthermore, operator interpretation is normally required to determine whether an explosive may be present. This limitation is significant, since large quantities of explosives may produce an image that is indistinguishable from an innocuous object.
The surface swipe involves an operator passing a clean swab or other collection medium over a vehicle surface, and then doing an on-site inspection for explosive particles that may have been collected by the medium. The surface swipe is not automated, so it has high labor costs; however, equipment costs are relatively moderate.
There are several types of nuclear detection systems. Typical systems include U.S. Pat. No. 5,124,554 of P. Fowler et al., Explosives Detector, which discloses a thermal neutron activation system based on the interaction of neutrons with nitrogen atoms in explosives. U.S. Pat. No. 5,098,640 of T. Gozani et al., Apparatus and Method for Detecting Contraband Using Fast Neutron Activation, discloses a technique based on the interaction of neutrons with explosive material, which results in the emission of gamma rays with characteristics that are indicative of the specific material. And U.S. Pat. No. 5,592,083 of E. Magnuson et al., System and Method for Contraband Detection Using Nuclear Quadrupole Resonance Including a Sheet Coil and RF Shielding via Waveguide Below Cutoff, uses nuclear quadrupole resonance excited by an RF signal to detect explosives. These systems are usually somewhat less expensive than x-ray equipment, but they are still not fast enough for heavy traffic checkpoints and they may suffer from false alarms caused by common materials. Also, quadrupole resonance may not be useful in vehicle screening applications due to the shielding effect of a metal vehicle body.
A relatively new technology that is being tested for detection of explosives on people is disclosed in U.S. Pat. No. 5,915,268 of K. Linker et al., Vertical Flow Chemical Detection Portal, which patent is owned by the assignee of this invention. This patent discloses an open-sided portal in which a flow of air is directed vertically over a person in the portal, and horizontal jets help dislodge particles from the person's body. The air is passed through a concentration apparatus to an ion mobility spectrometer for detection and identification of trace explosives.
One of the positive features of the portal detector is that trace explosives detected by this system do not occur naturally, and cleansing of detectable particles from a person or object that contacted explosives is not a trivial matter. Accordingly, if an automatic portal detector provides a positive indication, it may not mean that the object or person is presently carrying explosives, but it is a very reliable indication that they were recently in contact with explosives, and are a logical target for a search by trained people.
Although, conceptually, this desirable portal technology could be adapted to vehicles by having a larger drive-through portal, the amount of air movement that would be required to analyze a typical automobile would be very large, and implementation would require a garage or similar structure, making the device impractical to move.